A photodiode comprises a pn junction between semiconductor regions that are doped to have opposite types of electrical conductivity. A space charge region or depletion region forms at the pn junction. The extension of the space charge region and the strength of the electric field are increased by applying a bias voltage to the doped regions in the reverse direction, the voltage of the n-type region being positive with respect to the voltage of the p-type region.
When the photodiode is irradiated, electron-hole pairs are produced in the crystalline semiconductor material by lifting electrons from the valence band up into the conduction band, which allows a free movement of the electrons. The charge carriers thus generated in the space charge region produce a photocurrent, which can be detected in a control or read-out circuit connected to the photodiode. When the photodiode is not irradiated, the bias voltage generates a so-called dark current, which interferes with the measurement. The control circuit may be used to measure the dark current and to correct the measured value of the photocurrent accordingly.
U.S. Pat. No. 8,134,179 B2 discloses a photodiode comprising a pn junction between a doped region at the surface of a semiconductor substrate and a semiconductor layer of opposite type of conductivity arranged above the doped region. At the lateral margin of the doped region, a supplementary doping is applied to shift the pn junction and the space charge region to a position remote from the substrate surface. In this way charge carriers generated at crystalline defects near the substrate surface or at the edge of the doped region are prevented from contributing to the measured current.
U.S. Pat. No. 7,701,030 B2 describes a pinned photodiode formed by a region of n-type conductivity embedded in a region of p-type conductivity in a semiconductor substrate. The n-type region comprises a first zone and a second zone, which has a higher doping concentration and a smaller depth than the first zone. A shallow p+region is located at the surface above the n-type region. The doping concentrations are intended to deplete the n-type region during the operation of the device, which comprises an accumulation phase and a transfer phase. During the accumulation phase charge carriers are generated and stored in the n-type region; and in the transfer phase the charge carriers are driven through the transfer gate.
N. V. Loukianova et al., “Leakage current modeling of test structures for characterization of dark current in CMOS image sensors”, IEEE Transactions on Electron Devices vol. 50(1), pages 77 to 83, 2003, describe a photodiode comprising an n-well that is surrounded by a p-well in order to reduce the dark current. The lateral pn junction is passivated by a shallow p+implant, reducing the width of the space charge region at the surface by one half.
U.S. Pat. No. 7,312,484 B1 describes a photodiode sensor structure with a pn junction between an n-type doped well region and a p-type substrate, which is the anode of the photodiode. An electric shield is formed by a polysilicon region over substantially all of the diode junction. A sense node contact forming a cathode is located at a drain/source connection region between a cascode transistor and a reset transistor.